Lost wax investment casting was practiced in the days of Julius Caesar, but it is not known how or when the art was first developed. Jewelers have used the process through the past twenty one centuries, to cast small items of precious metals. In the last half century commercial foundries have turned to investment casting as a viable manufacturing process. Complex structural members of jet engines measuring several feet across are cast in exotic alloys in modern foundries.
A pattern, being a sacrificial model of the piece-part, is produced of wax, or a synthetic wax which is a plastic compound. In production, this pattern is usually produced by injection molding. If cores are required by the design of the part, then the cores are formed first using a wax laced with, for example, sodium bicarbonate. The pattern wax is then formed about the finished cores. Subsequently, the composite wax is submerged in a dilute acid bath which reacts with the additive in the core wax causing the cores to disintegrate without damage to the pattern. Gates and risers of wax, and a suitable pouring cup, are then attached using a hot wax compound which acts as a thermal-set glue. Smaller parts are generally clustered under one pouring cup and main riser, the assembly being known as a "tree".
The finished cluster is then "invested", that is to say that it is encased within a refractory mold. There are two types of refractory molds in common use, the solid mold, and the shell mold. There are also a number of less common systems in use.
Primary investment slurry is generally composed of very fine zircon sand along with a binder of colloidal silica or of hydrolized ethyl silicate. Other binders sometimes used include gypsum plaster, and potassium or sodium silicate.
Solid mold investment involves a single dip of the pattern cluster in refractory slurry, exclusive of the top of the pouring cup. The coating is allowed to dry. The cluster is positioned in a flask which is then filled with slurry and dried. Subsequently the entire flask is dewaxed, fired, and poured.
A ceramic shell investment is produced by dipping the pattern into a refractory slurry, then stuccoing the wet coating with sand, and allowing the coating to dry slowly. This sequence is repeated a number of times, the first dip or two using a slurry of zircon sand, and a zircon stucco. Subsequent backup slurries and stuccos generally rely on coarser refractories of fused silica or aluminosilicate to provide a denser and stronger shell.
The mold is then placed in a steam autoclave, being positioned with the pouring cup downward so that melted wax can completely drain from the mold. Steam provides the heat to melt the wax pattern, and pressure against the outside of the mold to support the mold against cracking as the heated wax within tends to expand.
The empty mold is transferred to a ceramic film where it is heated for several hours to eliminate all traces of moisture, and all traces of residual wax. The temperature is increased for another period, generally two or three hours, to vitrify the ceramic, making it very hard and strong, and incandescently hot.
The metal is melted, then the incandescent mold is removed from the kiln and placed on a bed of sand and immediately filled with metal. The metal quickly freezes, and as the solid metal cools the shrinkage partially breaks the mold. When cooled, the remaining investment is broken away, the gates and risers are cut off, and the finished casting is cleaned.
The investment casting process is a precision casting process. If the pattern is well made, no flash or parting lines are produced, and therefore need not be removed. Drafts are not usually required in investment casting, unlike sand casting which requires generous drafts. Investment casting is, however, a more expensive and labor intensive process.
"Full mold casting", or "Evaporative pattern casting" is a newly developing process based on a pattern formed of light plastic foam instead of wax. Most commonly, the pattern is made of expanded-bead polystyrene. Cores are not used. Instead, cavities are commonly formed by the use of "drawbacks", a trick practiced in the aluminum tooling used to produce the pattern. Another option is to make the pattern of multiple pieces which are joined together by the use of adhesive, leaving well formed cavities in between.
The use of polymeric foam as an evaporative pattern was first taught by Shroyer in U.S. Pat. No. 2,830,343. The use of an evaporative foam pattern in conjunction with unbonded sand was taught by T. R. Smith in U.S. Pat. No. 3,157,924.
The prepared pattern cluster is dipped, or otherwise coated, with a single coating of refractory slurry. The proper selection of refractory slurry is critical to the success of this casting process. The lengthy development of satisfactory refractory slurries kept the process from practical commercial use for years, consequently such refractory slurries are proprietary to their developers. The finished coating must be strong enough to support the internal pressure, weight, and erosive forces of the liquid metal; refractory enough to withstand the high temperature and thermal shock of the pour; must be gas permeable enough to vent the vapors from the evaporating pattern; and must be used "unfired", since firing would destroy the pattern.
The coated pattern cluster is dried in a low temperature, well ventilated, kiln (135.degree. F./145.degree. F. for about three hours, or until dry. The dried and coated pattern cluster is then positioned within a flask and the flask is packed with course dry unbonded foundry sand, and poured. The pattern fully occupies the mold at the start of pouring, hence the term full mold casting. At the liquid metal/pattern interface the pattern experiences a combination of melting, vaporization, combustion, and probably, sublimation. The voluminous byproducts of this process are vented through the permeable pattern coating and through the course sand beyond. The destruction of the pattern progresses at a rate dependent upon the pouring temperature, metalstatic head, and permeability of the composite mold. In the full mold casting process, flow of metal into the mold is controlled primarily by the permeability of the mold instead of the size of the ingates.
In other casting processes, the metal flows into the mold at a rate controlled by ingates and the risering system. In "full mold casting" the ingates are made as large as possible without interfering with the ordered destruction of the pattern as described above. The enlarged ingates are depended upon to lend strength to the pattern cluster during coating and handling. The plastic foam comprising the pattern is made a minimum density to reduce the volume of waste byproducts formed by pyrolyzation of the pattern. The foam at a typical density of 1.0 to 2.0 pounds per cubic feet, lacks the strength found in a wax pattern. At 1.0 pound density, the byproducts of decomposition are easiest to handle, but the pattern is more easily damaged and has lower dimensional stability. At 2.0 pound density, the strength and dimensional stability of the pattern are greatest, however so are the problems of handling the byproducts of decomposition. The large volumes of vapors have been known to fluidize the sand in the flask allowing the mold to burst and collapse at that unsupported location.
Three types of defect are peculiar to ferrous full mold casting. Metal flows coming from opposing directions may carry carbonaceous surface films which prevent the two masses from fusing properly thus producing a "lap" defect. Also, in heavier sections, masses of foam pattern may be trapped by freezing metal, and, being cut off from venting, show up as massive carbon inclusions or gas pockets in the finished casting. Part of the pattern which burns produces carbon soot which cannot vent through the pattern coating. This soot collects on certain surfaces of the casting forming pits and lustrous hard carbon deposits.
One of the economic advantages of the full mold casting system is that the unbonded sand may be reused several times with little or no recovery processing. Some of the decomposition byproducts do condense within the sand, cooling to solid organic contaminants affecting the reusability of the sand, and limiting the number of times the sand may safely be reused.
In the complex decomposition process, a number of noxious, toxic, or cancer causing compounds are produced which are of environmental concern. Compounds found in and around the mold are: styrene, toluene, benzene, ethyl benzene, methane, ethane, propane, hydrogen, and carbon soot.
The full mold casting process is being used in a growing number of large commercial foundries, casting aluminum and iron. Large foundries are making aluminum automotive inlet manifolds and cylinder heads, and cast iron exhaust manifolds, engine blocks, and other automotive components. Large commercial foundries producing valves, pumps, and pipe fittings are in operation.
At the pouring temperatures of iron and above, vapors of decomposing plastic foam either burns or "cracks" producing much free carbon soot which cannot escape from the mold. Iron is already saturated with carbon, but when cast in the full mold process, hard lustrous carbon deposits or carbon filled pits form on the surfaces of castings. Such defects, called "elephant skin" occur on sections of casting over about 15 mm thick. The direct control of such defects has been to avoid casting sections heavy enough to cause deposits. Polymethyl methacrylate foam has been introduced as an option to polystyrene foam, having the advantage of producing little free carbon upon pyrolysis. Ancillary methods of venting the offending carbon are also employed.
Low carbon steel cannot be successfully cast using the full mold casting process; the advantages of precision casting can be attained only through the substantially slower and more expensive lost wax process.